It is well known to use a combination spring and friction type vibration damper in a conventional vehicle friction clutch in the drive train between the vehicle engine and the manual transmission therefor. In more recent years with the addition of a lock-up clutch in a torque converter for an automatic transmission to provide direct drive from the vehicle engine to the automatic transmission without the normal slippage found in a torque converter, a vibration damper has been utilized in the lock-up clutch to overcome the undesirable characteristics which would be normally hydraulically absorbed in the torque converter.
More recently, series type vibration damper assemblies have come into more prominent use for the lock-up clutches in torque converters and for friction clutches for manual transmissions to provide a relatively low rate, high amplitude deflection between the driving and driven means in the vehicle drive train. Such an assembly includes a hub operatively mounted on the transmission input shaft and having two or more radial arms, an input member in the form of a driving plate with tangs or a housing for the damper with inwardly offset drive straps, one or more floating equalizers having radial arms or floating skates guided by the periphery of the housing, and damper springs located between the hub arms and the arms or skates of the floating equalizers. This assembly will provide two or more groups of damper springs between adjacent hub arms acting in parallel with the springs in each group acting in series.
A graph of the deflection curve for the above described vibration damper assembly discloses a three stage curve with the slope of each stage of compression indicating a more rapidly increasing torque for the degrees of travel of the damper. This three stage curve will be present in either the drive or coast direction of travel for the damper. Also, as seen in U.S. Pat. No. 4,188,805, spring deflection limiting members are utilized as stops to limit the compression of certain damper springs during damper operation, but the three stage deflection curve will still be present.
When the damper changes from a drive mode to a coast mode, the inertia in the damper as it returns from the three stage deflection curve previously described to the coast direction causes an undesirable "slingshot" effect in the drive train which will be evident to the vehicle operator when the accelerator pedal is released. To minimize this effect, it is desirable to bypass two of the three damper springs in the series for each group. The present invention provides a solution to the problem of the undesirable inertia resulting from the change from drive to coast travel.